I skimmed the 3 page thread that you linked to. Only the very first few posts discuss motorcycle oil and I did not see anyone recommending it for the purpose of particle control and smoother shifting. I did see that the user named Richard1 posted that motorcycle oil was good for Minis with automatic gearboxes.
I know nothing about the Mini and/or Austin America automatic gearbox so I cannot comment about that. The thread also took many sidetracks about filters, ZDDP, and other additives. On page 2 the character "Robster" (Tony something) shows up and the meaningful content in the thread evaporates.
Stick with a quality 20W50 with some level of ZDDP and your engine will be well protected. I use VR1 but that is a personal choice, yours may be different. As I mentioned earlier, you cannot prevent the particulates from forming so you want them kept in suspension long enough to be picked up by the pump and passed into the filter to be caught. Change your oil often. For me, this means once a year or 3k miles.